Like every metal monster, this old locomotive needed some extra dust, scratches and grease to look more realistic.
The first idea was to be used planar mapping from the front view. But the problem is coming when some objects cover other visible parts - in the final camera view with this simple mapping the user will have a lot of problems because of the texture overlapping.

When an artist grabs a tablet pen, he wants to paint over the entire picture, not only on wheels for example.
I used the same texturing method, but with a little trick:

1. Choose the parts that are on the same plane. Group the selection.

2. From the front viewport (to avoid perspective distortions) render the whole locomotive with white background in 1024x768 for example. Save the picture.

3. Clear the render window with X

4. Click one of the just created groups. Render the scene with 'render selection' option. Save the picture in .TGA format with alpha channel.

5. Open both of the pictures in Adobe Photoshop 5 or latest.

6. Use the picture of the whole locomotive as base picture and background.

8. In the menu Select choose Modify>Expand. Expand the selection with three pixels. In Edit>Copy copy the selected.

9. Click on the base image. Paste. Choose 'Difference' in the 'layers' floating window. Match the new layer to the background. If the detail becomes black the new layer is exactly where must be. Change 'Difference' with 'Normal'. Click on 'preserve transparency' to avoid painting over the empty areas. Go to MAX.

10. Render in the same way the other parts of the model and match them to the base picture.